Earth-boring tools for forming boreholes in subterranean earth formations, such as for hydrocarbon production, carbon dioxide sequestration, etc., generally include a plurality of cutting elements secured to a body. For example, fixed-cutter earth-boring rotary drill bits (also referred to as “drag bits”) include cutting elements fixed to a bit body of the drill bit. Similarly, roller-cone earth-boring rotary drill bits may include cones mounted on bearing pins extending from legs of a bit body such that each cone is capable of rotating about the bearing pin on which it is mounted. A plurality of cutting elements may be provided on each cone of such a drill bit.
Bit bodies may be formed of metal-matrix composites having a continuous phase and a dispersed phase. The continuous phase may be a metal or an alloy, such as a copper alloy, steel, cobalt, a cobalt-nickel alloy, etc. The dispersed phase may be a reinforcing material, and may be a different metal or another material, such as a ceramic. The dispersed phase may be selected to impart a particular property to the composite, such as hardness, wear resistance, strength, thermal conductivity, etc. For example, the dispersed phase may include materials such as tungsten carbide, cubic boron nitride, silicon carbide, diamond, etc. The dispersed phase may include, particles, fibers, whiskers, etc.
Metal-matrix composite materials may be formed by various methods. For example, a hard particulate material such as tungsten carbide may be placed within a graphite mold and infiltrated with a binder, such as a copper alloy. Cast resin-coated sand, graphite displacements or, in some instances, tungsten carbide particles in a flexible polymeric binder, may be employed to define topographic features of matrix-type bits. Because a composite material comprising tungsten carbide or other relatively hard particles may be substantially unmachinable, a machinable steel blank is typically disposed within the bit mold prior to infiltration of the hard particulate material, the steel blank forming a portion of the matrix-type rotary drag bit body upon hardening of the infiltrant that affixes the blank therein. In a manner similar to fabrication of steel body drill bits, the matrix-type bit body, via the machinable blank, may be secured to a hardened steel shank having a threaded pin connection for securing the bit to the drive shaft of a downhole motor or directly to drill collars at the distal end of a drill string rotated at the surface by a rotary table or top drive.
Bit bodies may also be formed by casting. For example, a solid material may be dispersed within a molten material, and the mixture may be cooled. As the molten material solidifies, it becomes the continuous (matrix) phase of the composite, whereas the solid material is the dispersed phase. In other methods, the material selected to form the matrix phase may be mixed, as a powder, with the material selected to be the dispersed phase. The mixture may be placed into a mold. The mixture may then be bonded by melting and re-solidifying the matrix phase, such as through pressing (e.g., hot isostatic pressing (HIP), uniaxial pressing), sintering, etc.
Conventional metal-matrix composite materials (and thus, bit bodies or tools formed from such materials) may have some desirable mechanical and thermal properties, but may suffer problems such as relatively poor fracture toughness, poor machinability, poor reparability, and/or low dimensional precision.